How very sad!! First I would get another Vet to check her out. If a vet doesn't know what the problem is it something is wrong with the vet. If nothing else he should have given her a appetite stimulant to get her eating.
This is not right. We cannot just let her fade away. In the meantime try moving her food and her to a private area away from big brother, sometimes little ones are intimidated, even by their brothers out of eating, due to the other cat taking over the dish and litter box.
Isolate her for now with her own room, food, water, and litter box until you can get her to another vet, who knows enough to search out the problem.
This needs to be done TODAY!! She is hanging on by a thread and no food in a little girl like that could mean major problems, like liver failure. Please, Please make an apt to get her help at a better Vet--even emergency vet if necessary. She can not wait.
Let us know later today what the doctor has to say,,,,,Please, K?
Worried right along with you--CML
Sorry, Marin, forgot to mention that she is probably also in need of IV fluids , since she is probably VERY dehydrated--that alone can be severe in a kitten. Vet should of at least given her fluids, Yikes!!!!!
.......Sorry, SJ, as you can see I am a bit obsessive when it comes to these things. Thought of o ne more thing, please make sure your kitten is kept warm. Warm bedding, warm room, or at least a heating pad under the blankets on low so as not to hurt her. She has very little fat therefore is very cold. Remember also cats will PURR even in distress. It is their way of de-stressing and calming down.
Ok , I think that is it. Just please get her to emergency before it is to late for your little girl